Manual pipettes for taking up and discharging precise quantities of liquid are well known in the art. A typical manual pipette is described in U.S. Pat. No. 3,766,784 to Walker. Such pipette includes a manually moved knob which is connected to a piston operating in a cylinder of a pipette barrel. To actuate the pipette the operator moves the knob a distance equal to the stroke of the piston to seat it at an intake position. Release of the knob causes drawing in of liquid. The liquid is discharged by again moving the knob past the intake position until the piston seats on a discharge stop. Walker further shows and describes the well known use of a disposable tip removably mounted to the pipette to receive the liquid and avoid both hand and pipette wetting by the liquid being processed.
The liquid quantities involved in dispensing with a pipette may vary, but often are quite small. Typical quantities may be of the order of 1 to 1,000 microliters in either fixed increments or variable ranges of 1-20, 20-100, 100-250, 250-1,000, 1,000-5,000 micro liters are common. Variations of any selected value for these liquid qualities may affect the tests for which the pipette is used and care must be taken to assure uniformity in the take-up and dispensation of liquids with a pipette.
With prior art manually actuated pipettes, undesirable variations are introduced in the liquid samples due to a number of causes of which the most significant are attributable to operator handling. These errors arise by virtue of the fact that mechanical action is supplied by the operator's hand as the source of energy to pick up and dispense small quantities of liquids. Different volumes are quite frequently dispensed by different operators using identical fixed pipettes or identical settings on variable pipettes. This error is of concern to the analyst who depends upon the accuracy of the results to indicate what medication need be administered.
For instance, in the depression of the pipette control knob, the operator's thumb is employed. This places a practical limit on the length of the stroke of the piston to that which is comfortable and suitable for the hands of most operators. For enhanced accuracy, however, particularly involving small volumes, it is preferred that the piston stroke be long with a small cylinder bore cross-section. Such longer stroke, however, cannot be conveniently accommodated by the stroke capacity of the operator's thumb without quickly causing operator fatigue.
Some operators develop, through practice, an impressive speed and repeatability in the use of a manual pipette. As a result, a particular operator may handle liquid samples in an accurate manner. Fatigue, however, frequently is likely to show up as a change in the accuracy or repeatability in the use of the pipette. For example, over an extended period of use, such as may be involved in a medical diagnostic test procedure, the depression of the control knob against a spring may not consistently result in precisely the same fill or take-up stroke.
In many pipettes depression movement of the piston is possible beyond an intake position stop to accommodate a longer discharge stroke and achieve a blow out feature of the previously taken up sample. A slight overshoot of the piston during the intake operation is likely to cause an accuracy error. Also, since the discharge stroke is made against the spring pressure, the discharge of the liquid sample may be but partially completed causing an error in the amount of liquid being dispensed.
The speed of the strokes also affects the accuracy of the manual pipette. Although the intake stroke occurs with the aid of a spring bias, the operator controls the speed by resisting the spring force--thus reducing the speed of intake as a function of operator "feel." An experienced operator may be capable of providing consistent speeds of intake and discharge strokes, but usually for limited periods below operator fatigue levels. Generally, regardless of the operator technique or speed, the speed and thus accuracy in the case of a conventional pipette, tends to vary. Hence, reliability of repeated or rerun procedures is compromised.
Although these operator errors may appear small, the errors are frequently considered too great for reliable comparison of diagnostic tests performed at different laboratories by different manual pipette operators. This frequently leads to unnecessary repeats of tests as well as a large number of tests to establish statistically reliable results. If greater consistency in the use of manual pipettes could be achieved, greater reliance upon laboratory test results can be placed.
Automatically operated pipettes of various types have been described in the prior art. In the U.S. Pat. No. 3,915,651 to Nishi, a digitally controlled pipette is described. The device dispenses small quantities of a liquid from a reservoir with a stepping motor which rotates a screw feed connected to a piston. Sample volumes may be delivered with an accuracy of the order of 0.2% for a 100 micro liter sample to 0.08% for an 800 micro liter sample. The Nishi pipette employs a stand mounted pipette whose operation is regulated by a separate controller. Such construction cannot be considered suitable to a portable hand-holdable application in which high dexterity is needed to perform rapid motions between wells on a tray used in a medical diagnostic tests or between more distant test stations. The construction of the Nishi pipette, furthermore, is not suitable to reach into test tubes.
The U.S. Pat. No. 3,719,087 to Thiers describes a manually controlled automatic pipette attached to a vacuum and pressure source by flexible tubes to respectively provide intake and discharge of liquid. The inaccuracy introduced in the pipetting of very small quantities with such device tends to be excessive and the device is not conveniently portable by virtue of a reliance upon flexible connecting tubes for actuation. Repeatability of this device is an eyeball affair. The unit is also location limited due to the use of air-vacuum lines.